MATLAB: Calling Matlab from C++: How to pass in no arguments in varargin

cMATLAB Compiler

I am compiling Matlab .m functions using Matlab compiler into libraries callable from C++. The particular function I am working with looks like this:

function status = cainit( model, varargin )
  if nargin==1
     % do something
  elseif nargin==2
     % do something else
  end
end

The C++ header file produced by Matlab compiler ("mcc") looks contains:

   extern LIB_libca_CPP_API void MW_CALL_CONV cainit( int nargout, mwArray& status, const mwArray& model, const mwArray& varargin );

I want to call cainit from my C++ code such that the nargin==1 part of the Matlab function gets executed. But I can't figure out how to pass in "no arguments".

For example, if I try the most obvious:

mwArray status;
cainit( 1, status, mwArray(""), mwArray() );

It still results in nargin==2 because mwArray() is an empty ([]) array.

Is there a way not to pass in an argument from C++? In other words, how do I do the C++ equivalent of Matlab "cainit('mymodel')"?

Thanks in advance!

Best Answer

Looks like this is not possible:

http://www.mathworks.com/help/compiler/matlab-compiler-generated-interface-functions.html

"Caution The C++ shared library interface does not support varargin with zero (0) input arguments. Calling your program using an empty mwArray results in the compiled library receiving an empty array with nargin = 1. The C shared library interface allows you to call mlfFOO(NULL) (the compiled MATLAB code interprets this as nargin=0). However, calling FOO((mwArray)NULL) with the C++ shared library interface causes the compiled MATLAB code to see an empty array as the first input and interprets nargin=1."

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MATLAB: How to call a MATLAB Compiler 4.4 (2006a) generated shared library function that has variable number of input arguments from the C++ application

The attached example files demonstrate calling a C++ shared library function which uses VARARGIN and VARARGOUT.

To execute the example code, perform the following steps:

1. Download the files vararg_example.cpp and makeplot.m to your current directory.

2. At the MATLAB prompt, compile the MATLAB file makeplot.m into a C++ shared library with the command:

 mcc -B cpplib:libplot makeplot.m

3. Compile the C++ program with the command:

 mbuild vararg_example.cpp libplot.lib

4. Execute the resulting EXE file from the MATLAB prompt:

 !vararg_example.exe

Alternatively, the EXE can be executed from the DOS prompt in the current directory:

 vararg_example.exe

The following modifications on "vararg_example.cpp" can be done to pass fewer arguments to the MAKEPLOT library function:

1. For passing no input arguments to MAKEPLOT, replace the following block of code:

mwArray input(1, 2, mxCELL_CLASS);
// Assign values to cell array "input":
mwArray range(1, 2, mxDOUBLE_CLASS);
range(1,1) = 5;
range(1,2) = 10;   // Range of data to be plotted.
mwArray title("Random numbers between 5 and 10"); // Plot title
input.Get(1,1).Set(range);
input.Get(1,2).Set(title);

with

mwArray input;

MATLAB: Converting std::vector type to mwArray

mwArray a(1, myvect.size(), mxDOUBLE_CLASS);
a.SetData(&myvect[0], myvect.size());

Best Answer

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